Mobile drive unit charging

ABSTRACT

Some examples include charging an onboard power source of a mobile drive unit (MDU) of an inventory system while the MDU is operating in the inventory system, rather than removing the MDU from service for recharging. As an example, the MDU may receive instructions to retrieve an inventory holder, and an onboard power source of the MDU may be charged while the MDU and the inventory holder are located at the charging station.

BACKGROUND

Modern inventory systems, such as those in mail-order warehouses, supplychain distribution centers, airport luggage systems, and custom-ordermanufacturing facilities, face significant challenges in responding torequests for inventory items. As inventory systems grow, the challengesof simultaneously completing a large number of packing, storing, andother inventory-related tasks become non-trivial. For instance, inproduct distribution centers (e.g., fulfillment centers), vastquantities of products are processed for shipment to consumerstraditionally using manual labor and/or mechanical handling equipment(e.g., conveyor systems, forklifts, etc.).

Inventory systems that utilize robots for managing inventory mayperiodically remove the robots from service in order to recharge therobots. As such, additional robots may be required in order tocompensate for the removal of some robots from service for recharging.Further, recharging stations may utilize valuable floor space that mayotherwise be used, for example, to store inventory.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is set forth with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical items or features.

FIG. 1 illustrates an example inventory system in which a mobile driveunit of an inventory system may be charged while operating, according tosome implementations.

FIG. 2 illustrates an example framework for charging a mobile drive unitof an inventory system while operating the mobile drive unit, accordingto some implementations.

FIGS. 3 and 4 illustrate components of an example mobile drive unitconfigured to charge while operating in an inventory system, accordingto some implementations.

FIG. 5 illustrates components of an example inventory holder for use inan inventory system, according to some implementations.

FIG. 6 illustrates select components of an example electronic device,according to some implementations.

FIGS. 7 and 8 illustrate example processes for charging a mobile driveunit while the mobile drive unit is operating in an inventory system,according to some implementations.

DETAILED DESCRIPTION

Inventory systems are utilized by many entities for storing and managinginventory. For example, some retailers may utilize a warehouse of racksthat store inventory items in various bins. When an order for a specificinventory item needs to be filled by the retailer, a worker typicallyretrieves the inventory item from the bin where the inventory item isstored.

Inventory systems of the present disclosure utilize one or more mobiledrive units to automatically retrieve inventory holders from warehousestorage. The inventory holders may be entire racks of shelves, with eachshelf having bins for various inventory items. Mobile drive units(“MDUs”) may be self-powered robotic devices configured to move freelyabout the warehouse. Racks may be transported by mobile drive units to apick station for performing inventory operations. The station mayinclude a human operator and/or automated equipment to remove desiredinventory items from the inventory holders and complete orders. Theability for mobile drive units to transport inventory items to a stationfor performing inventory operations rather than or in addition toworkers manually retrieving inventory items from stationary racks mayvastly increase efficiency and productivity of the inventory system. Forexample, by utilizing mobile drive units, an inventory system may becapable of fulfilling more orders per hour than previous solutions.Mobile drive units may move about the warehouse and/or retrieveinventory holders in response to commands and/or instructions receivedfrom an automated guidance system. For example, a management module maycontrol administration and coordination of certain elements and/or taskswithin the warehouse. The management module may receive orders forinventory items in the warehouse and coordinate task assignments forfulfilling the orders, including providing instructions to mobile driveunits to transport racks with requested inventory items to an inventorystation for completion of an order. The management module may alsoprovide guidance at the inventory station for how the order is to beassembled.

This disclosure includes, in part, techniques and arrangements forcharging a mobile drive unit while the mobile drive unit is operating inthe inventory system. In some implementations, one or more mobile driveunit charging stations at one or more inventory stations of theinventory system may allow the mobile drive unit to charge while themobile drive unit is docked with an inventory holder at or adjacent toan inventory station. In some implementations, one or more mobile driveunit charging stations within a storage area of the inventory system mayallow the mobile drive unit to charge while performing a task (e.g.,while presenting an inventory holder to an operator or while dockingwith or otherwise retrieving an inventory holder from storage) in theinventory system.

In some examples, the techniques described herein may reduce overallsystem cost by charging the mobile drive units while operating ratherthan removing the mobile drive units from service for recharging. As anillustrative example, over a particular operating time period, somemobile drive units may be recharged at an average rate of five minutesevery hour. In this example, approximately eight percent of all mobiledrive units would be removed from service for recharging at any giventime. In order to compensate for the removal of some of the mobile driveunits, the inventory system may rely on additional costly mobile driveunits.

The techniques described herein may reduce the number of mobile driveunits in an inventory system by reducing or eliminating the down-timeassociated with removing select mobile drive units from service forrecharging. Further, valuable floor space normally reserved for chargingstations may be freed up for alternative uses.

This and other examples of operating an inventory system are describedbelow. The techniques discussed herein may be implemented in manydifferent ways, by many different systems. Various representativeimplementations are provided below with reference to the figures.

FIG. 1 shows one illustrative example of an inventory system 100according to some implementations. The inventory system 100 may bearranged in a facility or warehouse (e.g., distribution facility,fulfillment center, etc.) that is logically organized into areas orregions associated with various functions. In FIG. 1, a mobile driveunit 102 may dock with an inventory holder 104 and move the inventoryholder 104 between locations within a workspace 106 (e.g., a storagearea of a warehouse). In some implementations, a management module 110may communicate with the mobile drive unit 102 to direct the mobiledrive unit 102 to retrieve a particular inventory holder 104 from aparticular location in the workspace 106. In the present disclosure, oneor more charging stations 112 within the inventory system 100 may allowan onboard power source (not shown in FIG. 1) of the mobile drive unit102 to be recharged while the mobile drive unit 102 continues to“operate” within the inventory system 100. To illustrate, in someimplementations, the mobile drive unit 102 may retrieve the inventoryholder 104 by docking with the inventory holder 104 and transporting theinventory holder 104 to an inventory station 108 that may include acharging station 112(1), in order to allow the mobile drive unit 102 tobe recharged at or adjacent to the inventory station 108. An operator atthe inventory station 108 may pick one or more inventory items from theinventory holder 104 or perform other inventory operations while themobile drive unit 102 is docked with the inventory holder 104 at oradjacent to the inventory station 108. That is, in this case, the mobiledrive unit 102 may continue to “operate” within the inventory system 100at or adjacent to the inventory station 108, rather than being removedfrom service for recharging at a separate charging location.Alternatively or additionally, in some implementations, while retrievingthe particular inventory holder 104 or otherwise operating at thestorage location, the mobile drive unit 102 may utilize a chargingstation 112(2) at the storage location to at least partially recharge anonboard power source. Thus, FIG. 1 illustrates an example inventorysystem 100 that may allow mobile drive units 102 to charge whileoperating (e.g., while loaded with inventory holders 104 at inventorystations 108 or while retrieving inventory holders 104 from storagelocations), rather than being removed from service for recharging.

The mobile drive units 102 may represent any devices or componentsappropriate for use in the inventory system 100 based on thecharacteristics and configuration of the inventory holders 104 and/orother elements of the inventory system 100. In some implementations ofthe inventory system 100, the mobile drive units 102 may representindependent, self-powered devices configured to freely move about theworkspace 106, under their own direction or through coordination by themanagement module 110. Alternatively, the mobile drive units 102 mayrepresent elements of a tracked inventory system 100 configured to movean inventory holder 104 along tracks, rails, or other guidance orsupport elements. It will be appreciated that the mobile drive units 102may be configured to utilize alternative conveyance equipment to movewithin the workspace 106 and/or between separate portions of theworkspace 106.

In some embodiments, the inventory system 100 may include two or moredifferent types of mobile drive units 102 having various capabilitiesand specifications. Moreover, although illustrated and discussed asthough mobile drive units 102 are a particular type of mobile driveunit, mobile drive units 102 may refer to various types of mobile driveunits. For example, one type of mobile drive unit 102 may be operable totransport relatively large, heavy, or bulky inventory items. Anothertype of mobile drive unit 102 may be operable to transport relativelylighter and/or more compact inventory items. Mobile drive units 102 mayaccordingly have various power trains, load capacities, and otherappropriate specifications to transport particular inventory items invarious types of the inventory holders 104 within the inventory system100.

The mobile drive units 102 may be capable of communicating with themanagement module 110 to receive information identifying selectedinventory holders 104, transmit the locations of mobile drive units 102,or exchange any other suitable information to be used by the managementmodule 110 or mobile drive units 102 during operation. The mobile driveunits 102 may communicate with the management module 110 wirelessly,using wired connections between mobile drive units 102 and managementmodule 110, and/or in any other appropriate manner. As one example, themobile drive unit 102 may communicate with the management module 110and/or with another mobile drive unit 102 using 802.11, Bluetooth, orInfrared Data Association (IrDA) standards, or any other appropriatewireless communication protocol. As another example, in a trackedinventory system 100, tracks or other guidance elements upon which themobile drive units 102 move may be wired to facilitate communicationbetween the mobile drive units 102 and other components of the inventorysystem 100. In general, mobile drive units 102 may be propelled andcontrolled in any manner appropriate based on the configuration andcharacteristics of the inventory system 100.

Each inventory holder 104 may be implemented as a physical structure tohold various inventory items. In FIG. 1, only some of the inventoryholders 104 are shown referenced with the number 104 for ease ofillustration. The inventory holder 104 has a physical length, width, andheight that may be standardized or varied within the inventory system100. As used herein, the inventory holders 104 may be configured to holdessentially any type or size of item or be used for any number ofpurposes, including, but not limited to, carrying pallets, storingshipping supplies, holding garbage, supporting empty boxes waiting forinventory, supporting filled boxes with items once orders are fulfilled,and so on.

In one implementation, the inventory holder 104 may be formed as a rackhaving multiple shelves to support various types of inventory items. Forinstance, the inventory holders 104 may include multiple storage binswith each storage bin capable of holding a different type of inventoryitem. The inventory holders 104 are capable of being carried, rolled, orotherwise moved by the mobile drive units 102. Each inventory holder 104may have a plurality of faces, and each bin may be accessible throughspecific faces. The rack is free-standing when at rest, but can belifted and moved by the mobile drive units 102. The mobile drive units102 may be configured to rotate inventory holders 104 at appropriatetimes to present particular faces of inventory holders 104 and theassociated bins to an operator or other components of inventory system100. One example is described below in more detail with reference toFIG. 5. While FIG. 5 illustrates one illustrative example of aninventory holder 104, it will be appreciated that alternative inventoryholder designs may also be used. For example, an inventory holder 104may have no shelves and be a “single container” (e.g., a bin or a totethat stores inventory items) that may be transported by the mobile driveunit 102. As another example, an inventory item may itself serve as itsown “inventory holder.” For example, if a car is lifted and moved by alarge mobile drive unit 102, the mobile drive unit 102 may be able toattach to the wheels of the car in order to lift the car. Further, itwill be appreciated that some mobile drive units may be capable ofretrieving inventory items directly from shelves rather than moving thewhole shelves.

The mobile drive units 102 may retrieve inventory holders 104 andtransport the inventory holders 104 between any number of predefinedphysical locations. For illustration purposes, rectangular areas aredepicted in FIG. 1 to designate physical locations within the facilitythat may be used to place an associated inventory holder. Each locationmay accommodate an inventory holder 104. That is, each inventory holder104 may stand at rest within the area of the floor reserved or otherwisepredefined as a storage location. An inventory holder 104 may be placedwithin a corresponding storage location until needed at one or moreinventory stations 108.

Inventory stations 108 represent locations designated for the completionof particular tasks involving inventory items. Such tasks may includethe removal of inventory items from inventory holders 104, theintroduction of inventory items into inventory holders 104, the countingof inventory items in inventory holders 104, the decomposition ofinventory items (e.g., from pallet-sized or case-sized groups toindividual inventory items), and/or the processing or handling ofinventory items in any other suitable manner. In some implementations,the inventory stations 108 may represent the physical locations where aparticular task involving inventory items can be completed within theworkspace 106. In alternative implementations, the inventory stations108 may represent both the physical location and also any appropriateequipment for processing or handling inventory items, such as scannersfor monitoring the flow of inventory items in and out of the inventorysystem 100, communication interfaces for communicating with themanagement module 110, and/or any other suitable components. Inventorystations 108 may be controlled, entirely or in part, by human operatorsor may be fully automated. Moreover, the human or automated operators ofinventory stations 108 may be capable of performing certain tasks toinventory items, such as packing or counting inventory items, as part ofthe operation of the inventory system 100.

In some implementations, the management module 110 orchestrates movementof the mobile drive units 102, directing them to various regions withinthe warehouse. The management module 110 coordinates transport of thevarious inventory holders 104 among the regions in the warehouse. Insome implementations, charging stations 112 may be associated withvarious locations in the warehouse. While FIG. 1 illustrates a singlecharging station 112(1) at one inventory station 108 and a singlecharging station 112(2) associated with one storage location in theworkspace 106, it will be appreciated that the inventory system 100 mayinclude multiple charging stations 112 at other locations within theworkspace 106 or at one or more additional inventory stations 108. As anillustrative example, in some implementations, each inventory station108 may include its own charging station 112.

In some implementations, the management module 110 may select aparticular MDU 102 to perform a particular task based at least in parton a charge level of an onboard power source of the MDU 102. Forexample, the management module 110 may determine that a particular taskto be performed at the inventory station 108 may include an operatorpicking multiple items from the inventory holder 104. The managementmodule 110 may determine a time period (e.g., an average “dwell time”)that an MDU 102 is expected to be docked with the inventory holder 104and located at the inventory station 108 in order for the operator toperform the picking task. Accordingly, in this example, the managementmodule 110 may select a particular MDU 102 to perform the task in orderto recharge the MDU 102 at the charging station 112(1) located at theinventory station 108 based at least in part on the charge level of theonboard power source of the particular MDU 102 and the expected dwelltime associated with the task. It will be appreciated that themanagement module 110 may select an MDU 102 with a higher charge levelto perform a task involving a shorter dwell time, while the managementmodule 110 may select a different MDU 102 with a lower charge level toperform a task involving a longer dwell time. In some implementations,the management module 110 may use multiple factors to select theparticular MDU 102 to retrieve the inventory holder 104, such as acharge level of the onboard power source, a time period associated withperforming a particular task, a location of an MDU 102 with respect tothe inventory holder 104, a location of an MDU 102 with respect to aninventory station 108, a location of the inventory holder 104 withrespect to an inventory station 108 or a relative speed of the operatorat the inventory station 108, among other alternatives. Further, themanagement module 110 may direct a particular MDU 102 to anotherlocation in the workspace 106 (or to another inventory station 108) thatincludes a charging station 112 and that is not currently being used, inorder to recharge the MDU 102.

Thus, the charging stations 112 of the present disclosure may notrepresent vacant stations dedicated to recharging that occupy valuablewarehouse space. Rather, the charging stations 112 may allow the MDU 102to be recharged while “operating” (e.g., when the MDU 102 is docked withthe inventory holder 104 at or adjacent to an inventory station 108 orat a storage location) as well as “not operating” (e.g., when theinventory holder 104 is not loaded onto the MDU 102). It will beappreciated that the term “operating” as used herein does notnecessarily indicate that all components of the MDU 102 remain active.Rather, select components of the MDU 102 that are not currently beingused may be powered down or otherwise deactivated during operation. Toillustrate, one or more components of the MDU 102 may be powered down orotherwise deactivated while the MDU 102 is docked with the inventoryholder 104 at the inventory station 108. In this illustrative example,the MDU 102 may still be considered to be “operating” at the inventorystation 108, as the MDU 102 has executed a retrieval operation involvingthe transportation of one or more inventory items to the inventorystation 108 via the inventory holder 104. The MDU 102 may remain at theinventory station 108 until the operator completes one or more taskssuch as picking items from the inventory holder 104 or replenishingitems on the inventory holder 104.

In some implementations, the mobile drive unit 102 may use a dwell timeassociated with the MDU 102 operating at a particular location toreceive a “quick burst” of charge that may allow for the MDU 102 toincrease its state of charge while waiting for the transaction to becompleted. As an illustrative example, the dwell time associated withthe MDU 102 retrieving the inventory holder 104 may average about twelveto fifteen seconds. Thus, on average, the MDU 102 may be able torecharge for an average of twelve to fifteen seconds while operating ata particular location. As another example, the dwell time associatedwith a picking operation at the inventory station 108 may vary based ona number of inventory items to be retrieved from the inventory holder104 while the MDU 102 is docked with the inventory holder 104 at theinventory station 108.

In some implementations, the average dwell time at a particular locationmay not provide an adequate recharge time period for particular types ofbatteries. That is, some batteries may not be able to absorb a largeamount of current quickly (e.g., hundreds of amps over a short dwelltime). As such, in some implementations, the MDU 102 may include acapacitor (e.g., an ultracapacitor or a supercapacitor) or other “fastcharging” device (e.g., particular types of lithium-ion batteries).

In some implementations, the MDU 102 may connect with a ground contactin or on a floor of the warehouse. In this case, there may be anelectromechanical linkage between the mobile drive unit 102 and thecharging station 112 (see e.g., FIG. 2). It will be appreciated thatalternative arrangements of one or more contacts (e.g., electricalcontacts on one or more sides of the MDU 102) may be used for electricalconnection with the charging station 112. Alternatively, a contactlesscharging system (e.g., an inductive charging system or other wirelesscharging system, such as a laser) may be used to connect the MDU 102 andthe charging station 112.

The illustrative workspace 106 of FIG. 1 represents an area associatedwith the inventory system 100 in which mobile drive units 102 can moveand/or inventory holders 104 can be stored. For example, the workspace106 may represent all or part of the floor of a mail-order warehouse inwhich the inventory system 100 operates. Although FIG. 1 shows, for thepurposes of illustration, an embodiment of the inventory system 100 inwhich the workspace 106 includes a fixed, predetermined, and finitephysical space, some implementations of the inventory system 100 mayinclude mobile drive units 102 and inventory holders 104 that areconfigured to operate within a workspace 106 that is of variabledimensions and/or an arbitrary geometry. Further, while FIG. 1illustrates an example inventory system 100 in which the workspace 106is entirely enclosed in a building, alternative embodiments may utilizeworkspaces 106 in which some or all of the workspace 106 is locatedoutdoors, on multiple floors or levels, within a vehicle (such as acargo ship), or otherwise unconstrained by any fixed structure.

In some examples, the management module 110 may generate taskassignments based, in part, on inventory requests that the managementmodule 110 receives from other components of the inventory system 100and/or from external components in communication with the managementmodule 110. These inventory requests may identify particular operationsto be completed involving inventory items stored or to be stored withinthe inventory system 100 and may represent communication of any suitableform. For example, in some implementations, an inventory request mayrepresent a shipping order specifying particular inventory items thathave been purchased by a customer and that are to be retrieved from theinventory system 100 for shipment to the customer. The management module110 may also generate task assignments independently of such inventoryrequests, as part of the overall management and maintenance of theinventory system 100. For example, the management module 110 maygenerate task assignments in response to the occurrence of a particularevent (e.g., in response to a mobile drive unit 102 requesting a spaceto park), according to a predetermined schedule (e.g., as part of adaily start-up routine), or at any appropriate time based on theconfiguration and characteristics of the inventory system 100. Aftergenerating one or more task assignments, the management module 110 maytransmit the generated task assignments to appropriate components forcompletion of the corresponding task. The relevant components may thenexecute their assigned tasks.

As part of completing these tasks, mobile drive units 102 may dock withand transport inventory holders 104 within the workspace 106. Mobiledrive units 102 may dock with inventory holders 104 by connecting to,lifting, and/or otherwise interacting with inventory holders 104 in anyother suitable manner so that, when docked, mobile drive units 102 arecoupled to and/or support inventory holders 104 and can move inventoryholders 104 within the workspace 106 and to the inventory stations 108.While the description below describes examples of a mobile drive unit102 and inventory holder 104 that are configured to dock in a particularmanner, alternative embodiments of mobile drive unit 102 and inventoryholder 104 may be configured to dock in any manner suitable to allowmobile drive unit 102 to move inventory holder 104 within the workspace106 and to the inventory stations 108.

Thus, FIG. 1 illustrates an example inventory system 100 in which themobile drive unit 102 may utilize the charging station 112 to at leastpartially recharge an onboard power source while “operating” at aparticular location (e.g., while docked with the inventory holder 104 ator adjacent to an inventory station 108 or while retrieving theinventory holder 104 from a particular location in the workspace 106).Allowing mobile drive units 102 to charge while operating, rather thanbeing removed from service for recharging at a dedicated rechargingstation, may reduce overall system costs associated with additional MDUs102 and may free up valuable floor space.

FIG. 2 illustrates an example framework 200 for charging the mobiledrive unit 102 while operating the mobile drive unit 102 in theinventory system 100 according to some implementations.

In FIG. 2, the mobile drive unit 102 is docked with an inventory holder104. That is, the inventory holder 104 is loaded onto the mobile driveunit 102, and the mobile drive unit 102 may transport the inventoryholder 104 between various locations in the warehouse. For example, themobile drive unit 102 may be directed to the particular inventory holder104 by the management module 110 of FIG. 1 in order to retrieve theinventory holder 104 that includes one or more inventory items and totransport the retrieved inventory holder 104 to an inventory station108. The mobile drive unit 102 may utilize one or more charging stations112(1), 112(2) to at least partially recharge at least one onboard powersource while the mobile drive unit 102 is operating in the inventorysystem 100.

In the example illustrated in FIG. 2, the mobile drive unit 102 includesa first power source 202 and a second power source 204. In someimplementations, the first power source 202 may include a “slow chargingdevice” (e.g., a battery), while the second power source 204 may includea “fast charging device” (e.g., a capacitor). In some implementations, afirst charging interface 206 (e.g., a first docking port) is associatedwith the first power source 202, and a second charging interface 208(e.g., a second docking port) is associated with the second power source204. However, in alternative implementations, it will be appreciatedthat the MDU 102 may include a single charging interface. In the exampleof FIG. 2, the MDU 102 may connect with a contact in a floor via aground contact 210. Alternatively, a contactless charging system (e.g.,an inductive charging system or other wireless charging system) may beused to connect the MDU 102 and the MDU charging station 112. In someimplementations, the second power source 204 (e.g., the capacitor) maybe used to recharge the first power source 202.

In some examples, the ground contact 210 may include one or more stripsin the floor that the MDU 102 drives over and makes contact with.Alternatively, there may be active engagement of the second charginginterface 208 with the ground contact 210.

FIGS. 3 and 4 illustrate in greater detail the components of the mobiledrive unit 102, according to some implementations. In particular, FIGS.3 and 4 include a front and side view of an example mobile drive unit102. The mobile drive unit 102 may include an end effector 302 (e.g., adocking head), a drive module 304, a docking actuator 306, and a controlmodule 308. As used herein, the term “end effector” may include one ormore components or mechanisms on the mobile drive unit 102 that aredesigned to dock with, lift, or otherwise assume control of theinventory holder 104 or the inventory itself. Additionally, the mobiledrive unit 102 may include one or more sensors configured to detect ordetermine the location of the mobile drive unit 102, the inventoryholder 104, and/or other appropriate elements of the inventory system100. In the illustrated example, the mobile drive unit 102 includes aposition sensor 310, a holder sensor 312, an obstacle sensor 314, and anidentification signal transmitter 316.

The end effector 302, in some examples of mobile drive unit 102, couplesthe mobile drive unit 102 to the inventory holder 104 and/or supportsthe inventory holder 104 when the mobile drive unit 102 is docked to theinventory holder 104. The end effector 302 may additionally allow themobile drive unit 102 to maneuver the inventory holder 104, such as bylifting the inventory holder 104, propelling the inventory holder 104,rotating the inventory holder 104, and/or moving the inventory holder104 in any other appropriate manner. The end effector 302 may alsoinclude any appropriate combination of components, such as ribs, spikes,and/or corrugations, to facilitate such manipulation of the inventoryholder 104. For example, in some implementations, the end effector 302may include a high-friction portion that abuts a portion of theinventory holder 104 while the mobile drive unit 102 is docked to theinventory holder 104. In such embodiments, frictional forces createdbetween the high-friction portion of the end effector 302 and a surfaceof the inventory holder 104 may induce translational and rotationalmovement in the inventory holder 104 when the end effector 302 moves androtates, respectively. As a result, the mobile drive unit 102 may beable to manipulate the inventory holder 104 by moving or rotating theend effector 302, either independently or as a part of the movement ofthe mobile drive unit 102 as a whole.

The drive module 304 propels the mobile drive unit 102 and, when themobile drive unit 102 and the inventory holder 104 are docked, maypropel the inventory holder 104. The drive module 304 may represent anyappropriate collection of components operable to propel the drive module304. For example, in the illustrated example, the drive module 304includes a motorized axle 318 (see FIG. 4), a pair of motorized wheels320, and a pair of stabilizing wheels 322. One motorized wheel 320 islocated at each end of motorized axle 318, and one stabilizing wheel 322is positioned at each end of mobile drive unit 102.

The docking actuator 306 may move the end effector 302 towards theinventory holder 104 to facilitate docking of the mobile drive unit 102and the inventory holder 104. The docking actuator 306 may also becapable of adjusting the position or orientation of the end effector 302in other suitable manners to facilitate docking. The docking actuator306 may include any appropriate components, based on the configurationof the mobile drive unit 102 and the inventory holder 104, for movingthe end effector 302 or otherwise adjusting the position or orientationof the end effector 302. For example, in the illustrated example, thedocking actuator 306 includes a motorized shaft (not shown) attached tothe center of the end effector 302. The motorized shaft may be operableto lift the end effector 302 as appropriate for docking with theinventory holder 104.

The drive module 304 may be configured to propel the mobile drive unit102 in any appropriate manner. For example, in the illustrated example,the motorized wheels 320 are operable to rotate in a first direction topropel the mobile drive unit 102 in a forward direction. The motorizedwheels 320 are also operable to rotate in a second direction to propelthe mobile drive unit 102 in a backward direction. In the illustratedexample, the drive module 304 is also configured to rotate the mobiledrive unit 102 by rotating the motorized wheels 320 in differentdirections from one another or by rotating the motorized wheels 320 atdifferent speeds.

The position sensor 310 may include one or more sensors, detectors, orother components suitable for determining the location of the mobiledrive unit 102 in any appropriate manner. For example, in someimplementations, the workspace 106 associated with the inventory system100 may include a number of fiducial marks that mark points on atwo-dimensional grid that covers all or a portion of the workspace 106.In such cases, the position sensor 310 may include a camera and suitableimage-processing and/or video-processing components, such as anappropriately-programmed digital signal processor, to allow the positionsensor 310 to detect fiducial marks within the camera's field of view.The control module 308 may store location information that the positionsensor 310 updates as the position sensor 310 detects fiducial marks. Asa result, the position sensor 310 may utilize fiducial marks to maintainan accurate indication of the location of the mobile drive unit 102 andto aid in navigation when moving within the workspace 106.

The holder sensor 312 may represent one or more sensors, detectors, orother components suitable for detecting the inventory holder 104 and/ordetermining, in any appropriate manner, the location of the inventoryholder 104, as an absolute location or as a position relative to themobile drive unit 102. The holder sensor 312 may be capable of detectingthe location of a particular portion of the inventory holder 104 or theinventory holder 104 as a whole. The mobile drive unit 102 may then usethe detected information for docking with or otherwise interacting withthe inventory holder 104.

In the illustrative example of FIGS. 3 and 4, the mobile drive unit 102includes an optional obstacle sensor. The obstacle sensor 314 mayrepresent one or more sensors capable of detecting objects located inone or more different directions in which the mobile drive unit 102 iscapable of moving. The obstacle sensor 314 may utilize any appropriatecomponents and techniques, including optical, radar, sonar,pressure-sensing and/or other types of detection devices appropriate todetect objects located in the direction of travel of the mobile driveunit 102. In some implementations, the obstacle sensor 314 may transmitinformation describing detected objects to the control module 308 to beused by the control module 308 to identify obstacles and to takeappropriate remedial actions to prevent the mobile drive unit 102 fromcolliding with obstacles and/or other objects.

The obstacle sensor 314 may also detect signals transmitted by othermobile drive units 102 operating in the vicinity of the illustratedmobile drive unit 102. For example, in particular implementations of theinventory system 100, one or more mobile drive units 102 may include anidentification signal transmitter 316 that transmits a driveidentification signal. The drive identification signal may indicate toother mobile drive units 102 that the object transmitting the driveidentification signal is in fact a mobile drive unit. The identificationsignal transmitter 316 may be capable of transmitting infrared,ultraviolet, audio, visible light, radio, and/or other suitable signalsthat indicate to recipients that the transmitting device is a mobiledrive unit 102.

Additionally, in some implementations, the obstacle sensor 314 may alsobe capable of detecting state information transmitted by other mobiledrive units 102. For example, the identification signal transmitter 316may be capable of including state information relating to the mobiledrive unit 102 in the transmitted identification signal. This stateinformation may include, but is not limited to, the position, velocity,direction, and the braking capabilities of the transmitting mobile driveunit 102. In some implementations, the mobile drive unit 102 may use thestate information transmitted by other mobile drive units to avoidcollisions when operating in close proximity with those other mobiledrive units.

The control module 308 may monitor and/or control operation of the drivemodule 304 and the docking actuator 306. The control module 308 may alsoreceive information from sensors such as the position sensor 310 and theholder sensor 312 and adjust the operation of the drive module 304, thedocking actuator 306, and/or other components of the mobile drive unit102 based on this information. Additionally, in some implementations,the mobile drive unit 102 may be configured to communicate with amanagement device of the inventory system 100, and the control module308 may receive commands transmitted to the mobile drive unit 102 andcommunicate information back to the management device utilizingappropriate communication components of the mobile drive unit 102. Thecontrol module 308 may include any appropriate hardware and/or softwaresuitable to provide the described functionality. In someimplementations, the control module 308 may include a general-purposemicroprocessor programmed to provide the described functionality.Additionally, the control module 308 may include all or portions of thedocking actuator 306, the drive module 304, the position sensor 310,and/or the holder sensor 312, and/or share components with any of theseelements of the mobile drive unit 102.

Moreover, in some implementations, the control module 308 may includehardware and software located in components that are physically distinctfrom the device that houses the drive module 304, the docking actuator306, and/or the other components of the mobile drive unit 102 describedabove. For example, in some implementations, each mobile drive unit 102operating in the inventory system 100 may be associated with a softwareprocess (referred to here as a “drive agent”) operating on a server thatis in communication with the device that houses the drive module 304,the docking actuator 306, and other appropriate components of the mobiledrive unit 102. This drive agent may be responsible for requesting andreceiving tasks, requesting and receiving routes, transmitting stateinformation associated with the mobile drive unit 102, and/or otherwiseinteracting with the management module 110 and other components of theinventory system 100 on behalf of the device that physically houses thedrive module 304, the docking actuator 306, and the other appropriatecomponents of the mobile drive unit 102. As a result, the term “mobiledrive unit” includes software and/or hardware, such as agent processes,that provides the described functionality on behalf of the mobile driveunit 102 but that may be located in physically distinct devices from thedrive module 304, the docking actuator 306, and/or the other componentsof the mobile drive unit 102 described above.

While FIGS. 3 and 4 illustrate a particular example of the mobile driveunit 102 containing certain components and configured to operate in aparticular manner, it will be appreciated that the mobile drive unit 102may represent any appropriate component and/or collection of componentsconfigured to transport and/or facilitate the transport of inventoryholders 104.

FIG. 5 illustrates in greater detail the components of a particularexample of the inventory holder 104. In particular, FIG. 5 illustratesthe structure and contents of one side of an example inventory holder104. In some implementations, the inventory holder 104 may comprise anynumber of faces with similar or different structure. As illustrated, theinventory holder 104 includes a frame 502, a plurality of legs 504, anda docking surface 506.

The frame 502 holds inventory items 510 and provides storage space forstoring inventory items 510 external or internal to the frame 502. Thestorage space provided by the frame 502 may be divided into a pluralityof inventory bins 512, each capable of holding inventory items 510. Theinventory bins 512 may include any appropriate storage elements, such asbins, compartments, or hooks.

In some implementations, the frame 502 includes a plurality of trays 514stacked upon one another and attached to or stacked on a base 516. Insuch cases, the inventory bins 512 may be formed by a plurality ofadjustable dividers 518 that may be moved to resize one or moreinventory bins 512. Alternatively, the frame 502 may represent a singleinventory bin 512 that includes a single tray 514 and no adjustabledividers 518. Additionally, in some examples, the frame 502 mayrepresent a load-bearing surface mounted on a mobility element 520. Theinventory items 510 may be stored on such an inventory holder 104 bybeing placed on the frame 502. In general, the frame 502 may includestorage internal and/or external storage space divided into anyappropriate number of inventory bins 512 in any appropriate manner.

Additionally, in some implementations, the frame 502 may include aplurality of device openings 508 that allow the mobile drive unit 102 toposition the end effector 302 adjacent to the docking surface 506. Thesize, shape, and placement of the device openings 508 may be determinedbased on the size, the shape, and other characteristics of theparticular mobile drive unit 102 and/or inventory holder 104 utilized bythe inventory system 100. For example, in the illustrated example, theframe 502 includes four legs 504 that form device openings 508 and allowthe mobile drive unit 102 to be positioned under the frame 502 andadjacent to the docking surface 506. The length of the legs 504 may bedetermined based on a height of the mobile drive unit 102.

The docking surface 506 comprises a portion of the inventory holder 104that couples to, abuts, and/or rests upon a portion of the end effector302, when the mobile drive unit 102 is docked to the inventory holder104. Additionally, the docking surface 506 supports a portion or all ofthe weight of the inventory holder 104 while the inventory holder 104 isdocked with the mobile drive unit 102. The composition, shape, and/ortexture of the docking surface 506 may be designed to facilitatemaneuvering of the inventory holder 104 by the mobile drive unit 102.For example, as noted above, in some implementations, the dockingsurface 506 may comprise a high-friction portion. When the mobile driveunit 102 and the inventory holder 104 are docked, frictional forcesinduced between the end effector 302 and this high-friction portion mayallow the mobile drive unit 102 to maneuver the inventory holder 104.Additionally, in some examples, the docking surface 506 may includeappropriate components suitable to receive a portion of the end effector302, couple the inventory holder 104 to the mobile drive unit 102,and/or facilitate control of the inventory holder 104 by the mobiledrive unit 102.

A holder identifier 522 marks a predetermined portion of the inventoryholder 104, and the mobile drive unit 102 may use the holder identifier522 to align with the inventory holder 104 during docking and/or todetermine the location of the inventory holder 104. In some examples,the mobile drive unit 102 may be equipped with components, such as theholder sensor 312, that can detect the holder identifier 522 anddetermine its location relative to the mobile drive unit 102. As aresult, the mobile drive unit 102 may be able to determine the locationof the inventory holder 104 as a whole. For example, the holderidentifier 522 may represent a reflective marker that is positioned at apredetermined location on the inventory holder 104 and that the holdersensor 312 can optically detect using an appropriately-configuredcamera.

FIG. 6 illustrates select example components of an electronic device 600(e.g., select components of the mobile drive unit 102 of FIGS. 1-4) thatmay be used to implement the functionality described above according tosome implementations. In a very basic configuration, the electronicdevice 600 includes, or accesses, components such as at least oneprocessor 602 and a computer-readable media 604. Each processor 602 mayitself comprise one or more processors or cores. The processor(s) 602can be configured to fetch and execute computer-readable instructionsstored in the computer-readable media 604 or other computer-readablemedia.

Depending on the configuration of the electronic device 600, thecomputer-readable media 604 may be an example of non-transitory computerstorage media and may include volatile and nonvolatile memory and/orremovable and non-removable media implemented in any type of technologyfor storage of information such as computer-readable instructions, datastructures, program modules or other data. Such computer-readable mediaincludes, but is not limited to, RAM, ROM, EEPROM, flash memory or othercomputer-readable media technology, CD-ROM, digital versatile disks(DVD) or other optical storage, magnetic cassettes, magnetic tape, solidstate storage, magnetic disk storage, RAID storage systems, storagearrays, network attached storage, storage area networks, cloud storage,or any other medium that can be used to store information and which canbe accessed by the processor 602 directly or through another computingdevice. Accordingly, the computer-readable media 604 may becomputer-readable media able to maintain instructions, modules orcomponents executable by the processor 602.

The computer-readable media 604 may be used to store any number offunctional components that are executable by the processor 602. In someimplementations, these functional components comprise instructions orprograms that are executable by the processor 602 and that, whenexecuted, implement operational logic for performing the actionsattributed above to the electronic device 600.

Functional components of the electronic device 600 stored in thecomputer-readable media 604 may include an operating system 606 forcontrolling and managing various functions of the electronic device 600.Depending on the type of the electronic device 600, thecomputer-readable media 604 may also optionally include other functionalcomponents, such as other modules 608, which may include applications,programs, drivers and so forth. The computer-readable media 604 may alsostore data, data structures, and the like that are used by thefunctional components. The electronic device 600 may also include otherdata 610, which may include, for example, data used by the operatingsystem 606 and the other modules 608. Further, the electronic device 600may include many other logical, programmatic and physical components, ofwhich those described are merely examples that are related to thediscussion herein.

FIG. 6 illustrates an implementation in which the electronic device 500includes one or more onboard power sources 612. For example, the one ormore onboard power sources 612 may include the first power source 202and the second power source 204 illustrated in FIG. 2. Further, theelectronic device 500 includes one or more charging interfaces 614. Forexample, the one or more charging interfaces 614 may include the firstcharging interface 206 and the second charging interface 208 illustratedin FIG. 2. While FIG. 2 illustrates a particular example of anelectronic device that includes two charging interfaces, it will beappreciated the electronic device 500 may include a single charginginterface or more than two charging interfaces. In some implementations,the first power source 202 includes a “fast charging device” such as anultracapacitor or lithium-ion battery, while the second power source 204includes a “slow charging device” such as a lead-acid battery.

One or more communication interfaces 616 may support both wired andwireless connection to various networks, such as cellular networks,radio, WiFi networks, short-range or near-field networks (e.g.,Bluetooth®), infrared signals, local area networks, wide area networks,the Internet, and so forth.

The electronic device 600 may further be equipped with various otherinput/output (I/O) components 618. Such I/O components may include atouchscreen and various user actuatable controls (e.g., buttons, ajoystick, a keyboard, a mouse, etc.), speakers, a microphone, a camera,connection ports, and so forth. For example, the operating system 606 ofthe electronic device 600 may include suitable drivers configured toaccept input from a keypad, keyboard, or other user actuatable controlsand devices included as the I/O components 618. For instance, the useractuatable controls may include page turning buttons, navigational keys,a power on/off button, selection keys, and so on. Additionally, theelectronic device 600 may include various other components that are notshown, examples of which include various sensors (see e.g., the sensorsof the example MDU 102 of FIGS. 3 and 4), removable storage, a globalpositioning system (GPS) device, a PC Card component, and so forth.

Various instructions, methods and techniques described herein may beconsidered in the general context of computer-executable instructions,such as program modules stored on computer storage media and executed bythe processors herein. Generally, program modules include routines,programs, objects, components, data structures, etc., for performingparticular tasks or implementing particular abstract data types. Theseprogram modules, and the like, may be executed as native code or may bedownloaded and executed, such as in a virtual machine or otherjust-in-time compilation execution environment. Typically, thefunctionality of the program modules may be combined or distributed asdesired in various implementations. An implementation of these modulesand techniques may be stored on computer storage media or transmittedacross some form of communication media.

FIGS. 7 and 8 illustrate example processes 700 and 800 for charging amobile drive unit while the mobile drive unit is operating in aninventory system, as described above. While FIGS. 7 and 8 illustrate theprocesses 700 and 800 as a collection of blocks in a logical flowdiagram, which represents a sequence of operations, some or all of whichcan be implemented in hardware, software or a combination thereof. Inthe context of software, the blocks represent computer-executableinstructions stored on one or more computer-readable media that, whenexecuted by one or more processors, perform the recited operations.Generally, computer-executable instructions include routines, programs,objects, components, data structures and the like that performparticular functions or implement particular abstract data types. Theorder in which the operations are described should not be construed as alimitation. Any number of the described blocks can be combined in anyorder and/or in parallel to implement the process, or alternativeprocesses, and not all of the blocks need be executed. For discussionpurposes, the processes are described with reference to thearchitectures and environments described in the examples herein,although the processes may be implemented in a wide variety of otherarchitectures or environments.

Referring to FIG. 7, at 702, the process 700 includes receiving, at anelectronic device that includes an onboard power source, instructions toretrieve an inventory holder (including one or more inventory items)from a particular location of an inventory system. As an illustrativeexample, referring to FIG. 1, the mobile drive unit 102 may receiveinstructions from the management module 110 to retrieve the inventoryholder 104 of the inventory system 100. At 704, the process 700 includesnavigating the electronic device to the particular location of theinventory system. For example, the mobile drive unit 102 may utilize themotorized wheels 320 (see the example MDU 102 of FIGS. 3 and 4) tonavigate to retrieve the inventory holder 104 from a storage locationwithin the workspace 106 of the inventory system 100.

At 706, the process 700 includes charging the onboard power source ofthe electronic device at a charging station while the electronic deviceand the inventory holder are located at the charging station. In somecases, the electronic device may remain docked with the inventory holderat the charging station for the duration of charging or may undock fromthe inventory holder for at least a portion of the charging time. Toillustrate, the mobile drive unit 102 may charge an onboard power sourceat one of the charging stations 112(1) or 112(2) of FIG. 1. FIG. 2illustrates an example in which the mobile drive unit 102 includes twoonboard power sources, the first power source 202 (e.g., a battery) andthe second power source 204 (e.g., an ultracapacitor). In someimplementations, the mobile drive unit 102 may charge the second powersource 204 by electromechanically coupling the mobile drive unit 102 tothe MDU charging station 112 via the second charging interface 208 andthe ground contact 210. Alternative methods of charging may also be used(e.g., induction coupling).

FIG. 8 illustrates another example process 800 for charging a mobiledrive unit while the mobile drive unit is operating in an inventorysystem.

At 802, the process 800 includes determining a charge level of anonboard power source of each mobile drive unit of a plurality of mobiledrive units of an inventory system. For example, while FIG. 1illustrates a single mobile drive unit 102, it will be appreciated thatthe inventory system 100 may include multiple mobile drive units 102that are remotely controllable by the management module 110, and themanagement module 110 may be configured to determine the charge levelassociated with each mobile drive unit 102. In some implementations, themanagement module 110 may communicate wirelessly with each mobile driveunit 102 to determine the charge level of the onboard source of eachmobile drive unit 102.

At 804, the process 800 includes selecting a mobile drive unit toretrieve an inventory holder from a particular location of the inventorysystem based at least in part on the charge level. For example,referring to FIG. 1, the management module 110 may select the mobiledrive unit 102 to retrieve the inventory holder 104 based at least inpart on the charge level of an onboard power source of the mobile driveunit 102. It will be appreciated that the management module 110 may usemultiple factors to select the particular MDU 102 to retrieve theinventory holder 104, including but not limited to the charge level ofthe onboard power source. For example, the management module 110 maydetermine a time period associated with performing a particular task, alocation of a mobile drive unit 102 with respect to the inventory holder104, a location of a mobile drive unit 102 with respect to an inventorystation 108, a location of the inventory holder 104 with respect to aninventory station 108, or a relative speed of the operator at theinventory station 108, among other alternatives.

As an illustrative, non-limiting example, the management module 110 maydetermine that a particular task to be performed at the inventorystation 108 may include an operator picking multiple items from theinventory holder 104. The management module 110 may determine a timeperiod (e.g., an average “dwell time”) that a mobile drive unit 102 isexpected to be docked with the inventory holder 104 and located at theinventory station 108 in order for the operator to perform the pickingtask. Accordingly, in this example, the management module 110 may selectthe mobile drive unit 102 in order to recharge the onboard power sourceat the charging station 112(1) located at the inventory station 108based at least in part on the expected dwell time associated with thetask. It will be appreciated that the management module 110 may selectthe mobile drive unit 102 with a higher charge level to perform a taskinvolving a shorter dwell time, while the management module 110 mayselect a different mobile drive unit 102 with a lower charge level toperform a task involving a longer dwell time.

At 806, the process 800 includes directing the selected mobile driveunit to retrieve the inventory holder. At 808, the process 800 includescharging the onboard power source of the selected mobile drive unitwhile the selected mobile drive unit and the inventory holder arelocated at a charging station.

The example processes described herein are only examples of processesprovided for discussion purposes. Numerous other variations will beapparent to those of skill in the art in light of the disclosure herein.Further, while the disclosure herein sets forth several examples ofsuitable frameworks, architectures and environments for executing theprocesses, implementations herein are not limited to the particularexamples shown and discussed.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Rather,the specific features and acts are disclosed as example forms ofimplementing the claims.

What is claimed is:
 1. A mobile drive unit comprising: one or moreonboard power sources; a communication module to receive instructions,from a management module of an inventory system, to retrieve aninventory holder from a particular location in the inventory system; adrive module to propel the mobile drive unit to at least the particularlocation of the inventory system using power from the one or moreonboard power sources; and a charging interface to couple at least oneof the one or more onboard power sources to a charging stationassociated with an inventory station to charge the at least one onboardpower source while (i) the mobile drive unit and the inventory holderare located at the inventory station and (ii) the mobile drive unit isexpected to remain stationary for an amount of time associated withcompletion of a user task associated with the inventory holder; whereinthe user task associated with the inventory holder comprises at leastone of removal or restocking of one or more inventory items from theinventory holder.
 2. The mobile drive unit as recited in claim 1,wherein the at least one onboard power source is electromechanicallycoupled to the charging station via a ground contact.
 3. The mobiledrive unit as recited in claim 1, further comprising: one or moreinduction components, wherein the at least one onboard power source iscoupled to the charging station via an induction coupling component thatis associated with the charging station.
 4. A method comprising: undercontrol of one or more processors of an electronic device that includesan onboard power source, the electronic device specifically configuredwith executable instructions to: receive instructions to retrieve aninventory holder from a particular location of an inventory system basedat least in part on a dwell time associated with a task to be performedwith respect to the inventory holder; navigate the electronic device tothe particular location responsive to receiving the instructions; andcharge the onboard power source at a charging station while theelectronic device and the inventory holder are located at the chargingstation and the task is performed.
 5. The method as recited in claim 4,further comprising navigating the electronic device including theretrieved inventory holder from the particular location to a differentlocation of the inventory system for charging the onboard power sourceat the charging station.
 6. The method as recited in claim 4, furthercomprising electromagnetically coupling the onboard power source to thecharging station.
 7. The method as recited in claim 4, furthercomprising coupling the onboard power source to the charging station viaa wireless coupling component.
 8. The method as recited in claim 7,further comprising coupling the onboard power source to the chargingstation via an induction coupling component.
 9. The method as recited inclaim 4, further comprising charging the onboard power source while theelectronic device is docked with the inventory holder at the chargingstation.
 10. The method as recited in claim 4, further comprisingcharging an onboard power source at the charging station; wherein theonboard power source includes a first onboard power source that iselectrically coupled to a second onboard power source of the electronicdevice.
 11. The method as recited in claim 10, further comprisingcharging the second onboard power source from the first onboard powersource.
 12. The method as recited in claim 10, further comprising:charging the first onboard power source at a first charge rate; andcharging the second onboard power source at a second charge rate;wherein the first charge rate is greater than the second charge rate.13. The method as recited in claim 12, wherein: charging the firstonboard power source includes charging a capacitor; and charging thesecond onboard power source includes charging a battery.
 14. The methodas recited in claim 12, wherein: charging the first onboard power sourceincludes charging a first type of battery; and charging the secondonboard power source includes charging a second type of battery that isdifferent from the first type of battery.
 15. The method as recited inclaim 14, wherein: charging the first type of battery includes charginga lithium-ion battery; and charging the second type of battery includescharging a lead-acid battery.
 16. An inventory system comprising: aplurality of mobile drive units remotely controllable to transport aplurality of inventory holders about a warehouse; a charging station tocharge an onboard power source of the plurality of mobile drive units;and a management module configured to: determine a charge level of theonboard power source of each mobile drive unit of the plurality ofmobile drive units; determine a dwell time associated with a task to beperformed with respect to an inventory holder of the plurality ofinventory holders while at the charging station; select a mobile driveunit of the plurality of mobile drive units to retrieve the inventoryholder based at least in part on the charge level of the onboard powersource of the mobile drive unit and the dwell time associated with thetask; direct the mobile drive unit to move the inventory holder from aparticular location in the inventory system to the charging station;wherein the charging station charges the onboard power source of themobile drive unit while the mobile drive unit and the inventory holderare located at the charging station and the task is being performed. 17.The inventory system of claim 16, wherein a first mobile drive unit ofthe plurality of mobile drive units has a higher state of charge than asecond mobile drive unit of the plurality of mobile drive units; whereina first task has a longer dwell time than a second task; and wherein themanagement module selects the second mobile drive unit to perform thefirst task based on a lower state of charge of the second mobile driveunit and the longer dwell time of the first task.
 18. The inventorysystem of claim 17, wherein the management module selects the firstmobile drive unit to perform the second task based on the higher stateof charge of the first mobile drive unit and a shorter dwell time of thesecond task.
 19. The inventory system of claim 16, wherein each of theplurality of mobile drive units further comprise at least one onboardpower source; and wherein the at least one onboard power source iselectromechanically coupled to the charging station via a groundcontact.
 20. The inventory system of claim 16, wherein each of theplurality of mobile drive units further comprises: at least one onboardpower source; and one or more induction components, wherein the at leastone onboard power source is coupled to the charging station via aninduction coupling component that is associated with the chargingstation.